In an update to its developer webpage ahead of Wednesday's iOS 9 release, Apple revealed that the soon to be replaced iOS 8 is currently running on 87 percent of compatible devices.

The statistic posted to Apple's App Store Support page shows 11 percent of devices are running iOS 7, while earlier iOS versions account for a two percent share. Apple drew its data from App Store visitations on Sept. 14.

Compared to the same time last year, when iOS 7 was about to be replaced by iOS 8, adoption for Apple's current-generation operating system is down three points. Despite booming iPhone sales and the release of Apple Music with iOS 8.4, uptake rates have been moderate. In April, Apple measured iOS 8 adoption at 81 percent.

After an official unveiling in June, Apple's iOS 9 is scheduled for release tomorrow. The new operating system will bring users a number of enhancements over iOS 8, including a tweaked UI, a News app, smarter Siri interactions, iPad-specific multitasking and Transit data for Maps, the latter being a much anticipated addition.

The new OS is designed to take full advantage of Apple's upcoming iPhone 6s and 6s Plus smartphones with support for pressure-sensitive 3D Touch finger input. Apple's 12.9-inch iPad Pro gets special attention with a subsystem for recognizing pressure- and tilt-sensitive Apple Pencil input, compatibility with Apple's new Smart Keyboard and support for big-screen multitasking.

Apple usually releases new iOS versions at around 10 a.m. Pacific, 1 p.m. Eastern, though an official launch window has not been provided for iOS 9.